The present invention relates in general to isotopic exchange by chemical route, for isotopic enrichment of uranium with respect to one particular isotope thereof.
As is known, isotopic enrichment of uranium has become or great importance and enrichment of natural uranium with respect to 235 U is of particular interest. Two methods of enrichment of natural uranium in isotope 235 are at the present time used industrially or are about to be so. These are gaseous diffusion and centrifugation. The two methods require the uranium to be in the form of the hexafluoride, that is to say in the form of a very corrosive gas, dangerous and difficult to handle. They are very complex and operation of the plants meets with considerable difficulties. The first method has additionally the drawback of a high consumption of energy. The second becomes of economic value only with a large inventory of centrifuges, requires enormous initial investments and is still not completely mastered.
On the other hand, there has been continuing interest in chemical methods of isotopic enrichment of uranium. Chemical exchange between ions of uranium of valence IV and uranium of valence VI, generally in the form of the uranyl ion, has been considerably studied. In particular, it has been proposed to effect istopic exchange between U.sup.+4 and UO.sub.2.sup.+2 in homogenous aqueous solution (for example in U.S. Pat. No. 2,787,587) or in aqueous and organic phases brought in contact (U.S. Pat. No. 2,835,687). It has also been proposed to use ion exchange resins which retain one of the isotopes selectively: for example, it has been proposed to fix uranium of valence IV on a resin, then to oxidize it and to elute it. On this subject, reference may be made to French Pat. Nos. 1,480,129, 1,600,437, 2,146,462 and 2,546,461.
While these methods have given some results, they have not achieved acceptance. First, the exchange factors per stage are low. Moreover, most of them require complex chemical operations.
Uranium is known to assume valences other than the valences IV and VI, which have been until now used in attempts of isotopic separation by the chemical route. SAITO has described in particular methods for the preparation of acid solutions of uranium salts of valence III (Bull. of the Chemical Society of Japan, 1967, Vol. 40, pp. 2107-2110). However, the U.sup.+3 ions tend spontaneously to revert, especially in solution, to the state of uranous U.sup.+4 ions or even uranyl UO.sub.2.sup.++ ions, which phenomenon has often been atributed to atmospheric air.
This re-oxydation in itself is not surprising if it is recalled that the system U.sup.+3 /U.sup.+4 forms an oxidation-reduction system whose normal potential is -0.63 volt with respect to a standard hydrogen electrode. The U.sup.+3 ions have consequently a reducing character such that they should theoretically reduce the water thereby causing the formation of hydrogen.
It is an object of the invention to provide an isotopic exchange method by the chemical route improved with respect to those defined above, especially in that it gives high enrichment coefficients per stage, that it only uses relatively conventional equipment, easy to operate, that it enables the use of non-gaseous phases, preferably liquids, that the consumption of energy that it involves compares favorably with that required by the majority of known methods and, lastly, that it can be carried out in low capacity installations.
The invention makes use of the observation that it is possible to form solutions containing uranium in the valance III state and in which this valence state can be preserved in metastable manner, even in acid medium, for long durations, in particular sufficient to perform isotopic exchanges under industrial conditions, when these solutions are kept out of all contact with conducting bodies and when they are practically free--apart from the uranium--of metal ions other than the alkali metals and alkaline earth metals (or of groups III to VIII of the periodic classification).
It is clear that, even if the normal potential of the oxidation-reduction system U.sup.+3 /U.sup.+4 mentioned above it left out from consideration, the species U.sup.+3 cannot escape its oxidation number rising by at least one unit under the effect of metallic ions with a less reducing character contained in their common solutions. It was however entirely unknown that quantities, even very small, of other metallic ions of the above-indicated type with respect to the content of U.sup.+3 ions of their common solutions had an effect which can be termed as catalytic with respect to the rapid oxidation of the U.sup.+3 ions.
It was noted that it is posible, for each type of metallic ion of groups III to VIII of the periodic table (other than uranium), to determine experimentally the minimum proportions, called below "catalytic proportions" beyond which the rapid conversion of the U.sup.+3 ions contained in the solutions concerned, into U.sup.+4 ions is observed. These catalytic proportions are very low, for example of the order of one ppm (part per million) with respect to the uranium, for ions such as nickel, copper or cobalt.
When, in the remainder of this description, reference is made to solutions free of metallic ions of the type concerned, it must be understood that it relates to solutions whose contents of these ions are less than the corresponding catalytic proportions.
Not only the phase containing U.sup.+3 must be practically free of certain ions, but any aqueous solution containing U.sup.+3 must be kept out of contact with the electrically conducting walls. If, for example, an acid solution containing U.sup.+3 is in contact with a conducting material, the latter can facilitate the reaction between U.sup.+3 and H.sup.+ (which is normally very slow in a pure homogeneous phase) by participating in the electron exchange between the two participants in the reaction. This phenomenon has a catalytic character and is more or less rapid according to the relative arrangement of the intensity-potential curves of the reactions, U.sup.+3 .fwdarw.U.sup.+4 and H.sup.+ .fwdarw.H.sub.2 in the material concerned. In particular, the phenomenon will be all the faster as the overvoltage of the hydrogen to this material is lower.
A process according to one aspect of the invention for effecting uranium isotopic exchange, comprises contacting uranium of valence state III and uranium of valence state IV, uranium being present in at least one of said valences in a liquid phase, under conditions which substantially prevent uranium of valence state III from rapidly oxidizing from valence state III to valence state IV.
In a preferred embodiment of the invention, the process comprises forming an aqueous phase containing hypo-uranous ions U.sup.+3, the contents of said aqueous phase in ions of metals of the groups III to VIII of the periodic table being however below the proportions which catalytically favor the oxidation of U.sup.+3 into U.sup.+4, and contacting said aqueous phase with an organic phase containing uranium in the valence state IV under conditions which exclude substantial transfer of uranium in either valence state from one phase into the other, while maintaining said aqueous phase out of contact with electrically conductive parts.
Particularly, the contents of the aqueous phase in any of the metals selected from the group consisting of nickel, copper or cobalt should be maintained below 1 ppm with respect to its content of U.sup.+3 ions.
According to another aspect of the invention, there is provided a process comprising digesting two compounds of uranium of valance state III which are not reactive with respect to each other, in a liquid phase and separating the two compounds.
It is not necessary to shield the phases and especially the aqueous phase from the atmospheric air, but in most cases, the method will be carried out in a closed installation, especially to avoid loss of solvents by evaporation.
In a particular embodiment, the process according to the invention is operated as a multistage process which comprises repeating several times, particularly a number of times sufficient to produce a substantial enrichment of the uranium in the 235 U isotope, and in a corresponding number of successive isotopic exchange sections, a cycle which comprises:
counter-current extraction by an aqueous phase previously depleted of its uranium contents, in a zone upstream of a given isotopic exchange section with respect to the direction of flow of said aqueous phase, of the U IV contained in an organic phase, which U IV already underwent an isotopic exchange in the said given section;
reduction of U IV, extracted as U.sup.+4 in the aqueous phase, and production within said given section of an isotopic exchange between the aqueous phase containing U.sup.+3 and the organic phase previously loaded with U IV, said isotopic exchange being effected under conditions which exclude substantial transfer of uranium in either valence state from one phase to the other, and
oxidation of U III into U IV within the aqueous phase, subsequent to the isotopic exchange, downstream of said section, and transfer of the oxidized uranium into the organic phase previously freed of its U IV contents.
Advantageously, substantial transfer of uranium IV into the aqueous phase from the other phase during the isotopic exchange operation is prevented by means of a salting-out agent (or relargant) of U IV previously introduced into the aqueous phase.
The uranium of valence IV may be extracted directly from the organic phase by the previously adjusted aqueous phase in order that the transfer may be practically complete; this adjustment can for example be constituted by a reaction in the content of salting-out agent. The extraction of U.sup.+3 from the aqueous phase may be done, after oxidation to U.sup.+4, by the organic phase selected so that the transfer is also practically complete for a suitable content of the aqueous phase of salting-out agent.
The aqueous phase which contains U.sup.+3 in solution must obviously in this case:
be able to contain U.sup.+4 ;
enable the extraction of uranium of valence IV by the organic phase and for this purpose be able to receive a salting-out compound which will generally be a halogen ion donor, the uranium then being present in the aqueous phase in the form of halogenide, whereby the oxidation and reduction operations are greatly facilitated, the hydrogen or hydracid obtained on reduction being used to effect the reoxidation. Particularly, the Cl.sup.- ion can be used as salting-out agent and UCl.sub.3 can be used as the uranium salt in the aqueous phase. Other non-reducing salting-out agents may however be used.
Obviously, the uranium contents of the aqueous and organic phases should be as high as possible; however, the following conditions should also be fulfilled and limit the contents: precipitation should not occur anywhere in the apparatus; the viscosity of the phases should be low enough not to impede the flow; the difference between the specific weights should be sufficient for separation to be easy.
In practice, for a complete reflux cascade, the flows of the organic phase and of the aqueous phase will be selected as a function of the uranium concentrations of the two phases, such that the flows of uranium into these two phases, be of the same order. The exchange conditions and particularly the content of salting-out agent of the aqueous phase will be selected so that less than 5% of uranium of valence IV passes from the organic phase to the aqueous phase. Practically passage of uranium of valence III from the aqueous phase to the organic phase does not occur in almost all cases, since there exist very few complexants of uranium of valence III.
The aqueous phase can only contain UCl.sub.4 in solution it if has a minimum acidity (which depends on the concentration of U.sup.+4 ions), failing which uranium precipitates in the state of the hydroxide. In practice however, it will often be possible to reextract UCl.sub.4 from the organic phase with water which is acidified by absorption of the acid contained in the organic phase.
As has been indicated above, the aqueous phase must contain a salting-out agent during contact with the organic phase. If it is assumed for simplification that this salting-out agent is constituted by Cl.sup.-, the U.sup.+3 containing phase must then contain a considerable concentration of hydrochloric acid or of a chloride. However, in the latter case, every chloride cannot be used. There must be avoided any cation:
which forms part of a Redox system whose standard potential is greater than that of the U.sup.+3 /U.sup.+4 system, if it reacts with noticeable speed, this condition having to be respected in homogeneous phase,
which has the above characteristics, if the second participant of the system is a metal. This metal will then be reduced by uranium and generally attacked again by the acid. A process of catalytic character will take place which will lead to rapid oxidation of U.sup.+3, even if the ion responsible for the process is present in very slight amount. This is particularly the case of nickel, copper or cobalt ions, the content of which must be kept less than 1 ppm (part per million) with respect to the uranium.
The phase containing U.sup.+3 must not only be practically free of certain ions, but any aqueous solution containing U.sup.+3 must be kept out of contact with electrically conducting walls. As a matter of fact when, for example, an acid solution containing U.sup.+3 is in contact with a conductive material, the latter can facilitate the reaction between U.sup.+3 and H.sup.+ (which is normally very slow in pure homogeneous phase) by participating in the exchange of electrons between the two participants in the reaction. This process of catalytic character is more or less rapid according to the relative disposition of the intensity-potential curves of the reactions U.sup.+3 .fwdarw.U.sup.+4 and H.sup.+ .fwdarw.H.sub.2 on the material concerned. In particular, this process will be all the more considerable as the hydrogen over-potential with respect to this material is less.
Summarizing, content of the aqueous solution with conductive materials must be avoided, except in very particular cases (cathode for electrochemical reduction kept under voltage) as well as the presence in the aqueous phase of ions which could catalyse its decomposition, in particular those having an oxidation-reduction potential comparable with that of nickel, copper and cobalt, even at very low contents. The only tolerable cations are those which cannot be reduced by U.sup.+3 such as ions of alkali metals or alkaline earth metals. The chloride containing salting out agents which are suitable in addition to HCl, are alkali chlorides or alkaline earth chlorides which are sufficiently soluble (such as LiCl or MgCl.sub.2).
The bringing of the valencies III and IV in contact can be effected by extremely various methods. The phases containing the uranium under the two valences can be mixable, partially miscible, or non-miscible. One of the phases (or the phase in the case of homogeneous phase) will be liquid. The other phase can be liquid or solid, especially ion exchange resins in the latter case.
The liquid phase, or each of the liquid phases, can be aqueous, organic or mixed, containing uranium in the form of ions or in the state of a complex (this case being often that of an organic phase). Solvents which are capable of solubilizing uranium IV are well known.
By aqueous phase, will be understood an aqueous solution of a mineral or organic salt of uranium in the dissociated state. The aqueous phase containing U.sup.+3 will generally be a solution of a hydracid whose halogen ion will play the role of a salting out agent. In any case, only non-oxidizing acids will be used. In practice, hydrochloric acid solution will generally be used, HCl being the least expensive of the strong acids, although use of other hydracids and, to a lesser extent, of non-oxidizing strong acids can be contemplated.
By organic phase, will be understood a solution in an organic solvent (or a mixture of such solvents) of a salt or complex of uranium, of valency III or IV, as the case may be, in addition a diluent as, for example, when it is desired to modify the viscosity, the density and/or surface tension of the organic phase and to act on various parameters, such as decantation times. Many liquid organic solvents may be used, and the technician skilled in the art will have no difficulty in selecting a suitable solvent. Recourse will generally be had to the well known solvents which are used in the treatment of irradiated nuclear fuels. These organic solvents will, as a general rule, be selected from the list below, depending on the selected uranium salt; it will often be necessary to add thereto a diluent which may be selected among aliphatic or aromatic hydrocarbons and their derivatives (such as benzene, toluene, dodecane, kerosene, xylene . . . ) which are liquid at ambient temperature. The solvents may be for example organic compounds belonging to the families below, which do not contain oxidizing impurities, and which are selected with regard to their high exchange capacity, their good resistance to hydrolysis and to their ability to allow for an easy decantation:
alcohols
anionic exchangers, such as tricaprylmethylammonium chloride (sold under the trade mark "Aliquat 336" by General Mills, Kankakee, Ill.)
neutral organophosphorous compounds, bearing the function P.dbd.0 which gives with uranium salts complexes generally including several ligands for one molecule of uranium. Several families of these may be distinguished:
phosphates of the type (RO).sub.3 P(O) or RO R'OR"OP (O) (the R,R' and R" radicals being linear or branched aliphatic or aromatic carbon chains of which two do not contain more than 8 and of which the third can extend up to 12 carbon atoms (the number being however preferably 6 at the most for each), for example: tributylphoshate (TBP), triisobutylphosphate (TiBP), tripropylphosphate (TPP), triethyl-12-butylphosphate (TEBP), tri-2-methylbutylphosphate (T2MBP), tri-2-ethylbutylphosphate (TBEP);
the phosphine oxides RR'R"P(O) in which the radicals are of the same family as previously; among the phosphine oxides, there may be indicated a particular family in which one of the chains bears the ether oxide function and of which the general formula is RO(CH.sub.2)n-P (O)R'R"; R, R' and R" are radicals as above and n is an integer at least equal to 1: trioctylphosphine oxide (TOPO), tributylphosphine oxide (TBPO), di-N-propylmethoxyoctylphosphine oxide, di-N-butylethyl-2-methoxyisobutylphosphine oxide, di-isobutylmethoxymethyloctylphosphine oxide, triamylphosphine oxide (having however the drawback of being soluble in water), trihexylphosphine oxide;
the phosphonates of formula ROR'OR"P(O) which constitute intermediate compounds, R, R' and R" having the above indicated meanings, such as: dibutylbutylphosphonate (DBBP), di-isobutylphosphonate, di-octyloctylphosphonate, di-isobutylbutylphosphonate, dibutylisobutylphosphonate, di-isoamylamylphosphonate, di-isopropylbutylphosphonate, di-isobutylhexylphosphonate, di-isobutylisoamylphosphonate, di-isobutyloctylphosphonate, di-isobutylethylhexylphosphonate and di-isobutylmethoxylaurylphosphonate; these phosphonates may be diluted for example by dodecane and/or xylene;
the phosphinates of formula ROR'R"P(O) which are also intermediate compounds between the phosphates and the oxides of phosphine, R, R' and R" having always the same meaning; among the phosphinates may be mentioned: di-isobutylphosphinate (used diluted in toluene), dihexylhexylphosphinate (used diluted in kerosene R), dibutylisobutylphosphinate (used diluted in toluene), di-isobutylbutylphosphonate (used diluted in toluene), dihexylisobutylphosphinate (used diluted in kerosene R), dioctylisobutylphosphinate (used diluted in kerosene R).
At the present time, the phosphates and phosphonates seem to be the solvents which give the best results if one takes into account the various factors which come into play (speed of extraction, facility of separation, etc.).
When working by liquid-liquid exchange between two phases, they must obviously be selected as a function of one another.
By solid organic phase, is meant any organic ion exchanger having fixed ionic compounds of uranium, such as ion exchange resins. Strong cation exchange resins are known (for example sulfonic polystyrene resins) weakly cationic complexing or chelating agents (for example resins bearing carboxylic, phosphate or aminodiacetic groups), strong anionic resins (for example quatenary ammonium), moderate or weak anionic resins (for example various amines). The above expression may also mean a mineral or organic compound deposited or adsorbed on a solid organic support (polystyrene, PTFE or Kel F for example) or a solid organic compound used alone.
Isotopic exchange by the method according to the invention can take place between two different compounds of uranium III or between compounds of uranium III and uranium IV: it is the latter solution which is by far the most interesting.
Isotopic exchange by the method according to the invention can take place either in a monophase liquid system, that is to say the uranium compounds are brought in contact in a homogeneous phase, or in a two-phase system, that is to say that the uranium compounds are in two different phases, like a liquid and a solid or two liquid phases which are not miscible. However when the isotopic exchange is effected in a monophase system, it is necessary to separate, after enrichment, either the depleted compound, or the enriched compound by creating a two-phase system, which complicates the method and renders the yield less advantageous.
Among the methods of applying the invention, exchange in liquid phase seems particularly advantageous, especially because it is possible to obtain and to maintain the necessary purity without excessive difficulty. Among these methods, those which seem most advantageous at the present time use exchange between an aqueous phase containing U III and an organic phase containing U IV. U III will generally be in the form of a salt dissociated in solution. The salt will for example be UCl.sub.3 in an aqueous solution of HCl playing the role of a relargent or salting-out agent.
The concentration of uranium of the aqueous and organic phases of the solutions is adjusted as a function of the compounds of uranium used, of their crystallisation limits, of the temperature of the clogging limit of the contactor selected. It will be chosen as high as possible to reduce the volume of the installation, but it must, as indicated above;
limit the passages of uranium with the valence IV from the organic phase to the aqueous phase in the course of the contact, the passage of uranium of valence III into the organic phase being negligible with the usual solvents;
enable the almost complete re-extraction by means of simple adjustments (elimination of the salting-out agent for example).
In practice, there is generally used for the exchange:
an 0.1-2.5 M/l aqueous solution of U III; with UCl.sub.3 in a hydrochloric solution, upper limit is preferably limited to 2 M/l;
a concentration of 1.5 M/l gives good results;
an 0.1-1 M/l organic phase of U IV. With UCl.sub.4 it will be possible in practice to arrive at 0.5 M/l at ambient temperature, by using the abovementioned complexants. The contents in the neighborhood of 1 M/l require in general working at higher temperatures than ambient, thereby reducing the proportion of diluent.
An isotopic exchange device or installation for enriching uranium in one of its isotopes, according to another aspect of the invention, comprises:
an exchange battery constituted by a plurality of stages each comprising a contactor between two phases, one containing U III, the other U IV, and means for causing the circulation of one of the phases in counter current with the other in the battery,
an oxidizing reflux circuit comprising means for performing substantially complete extraction of U III from the phase which contains it at the end of the battery whence this phase emerges, means for oxidizing U III to U IV and for transferring the oxidized uranium into the other phase for introduction at the same end of the battery,
a reducing reflux circuit comprising means for substantially complete extraction of U IV from the phase which contains it at the other end of the battery, for substantially complete reduction of U IV to U III and for transfer of the reduced uranium into the other phase for introduction at said other end, the whole of the surfaces in contact with U III being electrically insulating.
The means for reducing U IV to U III are advantageously provided to trap the troublesome ions: thus, the necessary purity is maintained provided that the content in these ions of the uranium solutions to be enriched introduced in the system be less than ppm and that the reflux be almost complete, that is to say the cascade almost square.
The uranium can be brought to the cascade at the valence III or IV: but in most of cases, the initial loading of the cascade will involve making use of particular compounds of U III and U IV.
Numerous methods for the preparation of uranium compounds with valence IV are already well known; there may be mentioned the electrolytic reduction of uranyl salts, the chemical reduction of these salts with a suitable reducer not permitting U.sup.+3 to be obtained (hydrogen or cracked ammonia without impurities), direct attack of a uranium oxide (UO.sub.2 by carbon tetrachloride towards 600.degree. C.) or of metallic uranium by an acid, followed by filtration.
Uranium III may be obtained from uranium IV or from its compounds by methods which will be described below with regard to reducing refluxes. There may for example be used reduction of uranium IV by the electrolytic route, by the chemical route (for example by zinc or its amalgam); uranium metal may also be attacked with an acid under the specified conditions; or by dissolving UCl.sub.3 (or other salts of U with valence III) obtained by a dry route.
It is lastly necessary to note that reduction with zinc of a solution of a uranyl salt with a suitable acidity 5 N hydrochloric acid for example) enables the obtaining of an equimolecular mixture of uranium III and IV which must then be protected against oxidation. In this case, isotopic exchange, in a monophase system, is produced at the same time as the formation of the two compounds.
Lastly, various examples of apparatus usable in the installations will be described with regard to the various embodiment envisaged below. It is however important to note that the speeds of isotopic exchange are very high and that, in consequence, the times of contact must be chosen as short as possible, to reduce the volume of the solutions, especially in the stages or the cascades where the uranium is very enriched. As a result, the use of contact apparatuses involving short transit times is advantageous in the case of liquid-liquid exchange. It may be noted by way of indication that usual mixer-decanters hardly enable dropping below 40 sec. and pulsed columns rarely below 30 sec., which leads to the preference of other equipment such as centrifugal mixer decanter assemblies, unfortunately more expensive.